The cornea is a thin clear layer of tissue that covers the eye like a window or like a crystal covering a clock face. The human cornea is composed of 3 layers, the outer or epithelial layer, the middle or stromal layer (which comprises about 90% of the total corneal thickness), and the inner or endothelial layer. The cornea plays an important role in vision by focusing light that comes into the eye. When the cornea is damaged either by injury, disease, or hereditary conditions, it may become swollen (edematous) or scarred. These scars and/or swelling can cause the light coming into the eye to scatter rather than focus, which results in decreased vision. All 3 layers can be individually or collectively affected by disease.
After completing the requirements for certification in general ophthalmology, Dr. Blair did additional training to become a cornea sub-specialist to treat diseases of the cornea, both medically and surgically.
Keratoconus is a progressive condition, often appearing in the late teen or early twenties, where the cornea thins and changes to form a cone-like shape. This irregular shape causes light coming into the eye to be distorted, resulting in blurred vision. While the rate of progression varies, the condition may progress slowly for 10 to 20 years before the progression stops. One eye or both eyes may be affected.
The cause of keratoconus is unclear. A genetic link may exist as an estimated 10 % of people with keratoconus also have a family member with the condition. Symptoms include:
In advanced keratoconus, typical changes associated with the thinning cornea can be visualized during an eye exam. In early cases, these changes are not evident. When keratoconus is suspected in these instances, a machine called a corneal topographer can produce a colored map showing the typical irregular shape of the cornea.
In mild forms of keratoconus, glasses and contact lenses are prescribed. As the condition progresses, rigid gas permeable contact lenses are the recommended treatment methods. If keratoconus reaches the point where an individual can no longer tolerate a lens, or contact lenses no longer provide good vision, surgical treatment may be recommended. Corneal transplants are very successful in keratoconus. New research utilizing riboflavin and uv light to cross-link the collagen fibrils in the cornea (thereby strengthening the cornea and slowing progression) is promising as a future treatment for keratoconus.
FUCHS' ENDOTHELIAL DYSTROPHY
Fuchs' dystrophy is an inherited condition involving the inner layer of the cornea, known as the endothelial cells. Usually occurring after age 40, the disease reduces the number of endothelial cells and produce abnormal outgrowths (guttata) from the inner surface of the cornea. The corneal endothelial cells actively pump water out of the cornea to maintain thinness and clarity of the cornea. As Fuchs' dystrophy progresses, the decrease in the number of endothelial cells result in a decreasing ability to keep up with the workload of pumping water out of the cornea, which then leads to swelling or edema of the cornea.
Initially, the disease may produce no symptoms or only mild symptoms. In the early stage, corneal swelling is more noticeable in the morning because closing your eyes during sleep keeps moisture from evaporating out of the cornea. The swelling and resulting blurred vision tends to clear later in the day initially, but as the disease progresses may remain blurred longer and longer throughout the day. It can take 10 to 20 years or longer for Fuchs' dystrophy to progress from the early to the late stage.
The treatment for Fuchs' dystrophy varies according to the severity of the disease. Initially, concentrated saline eye drops may be used to draw the swelling out of the cornea. In the later stages of the disease, if reduced vision begins to interfere with daily living, a corneal transplant, to replace the cloudy cornea with a donor's clear cornea, may be needed. See the section below on DSAEK for more information about this procedure.
Of all the transplant surgery done today - including heart, lungs, and kidney - corneal transplants are by far the most common and successful. Most surgeons who transplant corneas have additional training in cornea surgery, since the techniques, risks and complications are not commonly seen in general ophthalmology. Dr. Blair is such a specialty trained surgeon.
Conditions that may require a corneal transplant include:
Once it is decided by you and your doctor that a corneal transplant is needed, your name is placed on a list with the Arkansas Eye Bank. There are strict guidelines in obtaining corneal tissue from a donor and before the cornea is released for transplant, the eye bank tests the human donor for viruses that cause hepatitis and AIDS, as well as other entities. Unlike other organ transplants, studies show that tissue typing for matching between the donor and recipient does not decrease the chance of rejection.
During the surgery, the diseased or injured cornea is carefully removed from the eye and the clear donor cornea is sewn in place with nylon thread that is the thickness of a human hair.
Postoperatively, patients are watched carefully for any signs of rejection. Rejection can occur anytime after the surgery, even years later. The cornea is unique from other organs that are transplanted in that, in most cases, prompt treatment of the rejection can be done with steroid eye drops rather than with systemic immunosuppressive drugs that can lead to other complications.
Warning signs of a rejection are:The vision is slower to improve than after cataract surgery and may take up to a year or so to get the best vision. No other surgery has so much to offer in giving back sight that is lost when a cornea is deeply scarred or swollen. Of course, cornea transplant surgery would not be possible without the thousands of generous donors and their families who have donated corneal tissue so that others may see. Dr. Blair would like to encourage everyone to sign up to be an organ donor.
For the past 50 years, full-thickness corneal transplant (penetrating keratoplasty or PK) that replaces all three layers has been the standard of care for patients with corneal endothelial disease and has changed very little over that time. PK can produce clear corneas and has an overall 90% success rate, but it may take up to 1 to 2 years to see well and vision may not ever be completely clear because of irregularity of the surface and astigmatism. In addition, sutures remain in place a long time, and a ruptured wound is always a serious risk because the 360 degree full-thickness wound never heals as strong as before surgery.
Recently, however, cornea specialists are doing a new procedure known as DSAEK (Descemet's Stripping Automated Endothelial Keratoplasty) for certain corneal diseases where the inner or endothelial layer is abnormal. The endothelial cells are responsible for pumping fluid out of the cornea so it can remain clear and thin and provide good vision for the eye. If the pump cells should become dysfunctional or if a critical number of endothelial cells are lost, damaged, or destroyed, the cornea fills up with fluid and becomes swollen and cloudy, and causes blurry vision. The endothelial cells can be lost due to aging, from inherited diseases (such as Fuchs' corneal dystrophy), from trauma, or from previous intraocular surgery.
In DSAEK, only the dysfunctional endothelial layer is replaced, instead of all 3 layers being replaced. A thin button of donor tissue containing only the endothelial layer is inserted onto the back surface of the patient's cornea.
This new technique appears to be a significant improvement over the standard operation. The surgery involves a smaller surgical incision, requires far fewer sutures, heals faster and more reliably, and the vision returns faster. Since only the thin layer of the cornea is replaced, over 90% of the patient's own cornea remains behind contributing to greater structural integrity and a reduced incidence of rejection.
Since corneal specialists have only been performing DSAEK for the past 2 to 3 years, there is no long-term follow-up. But studies thus far are showing this technique to be superior to traditional penetrating keratoplasty for those corneal diseases resulting from abnormalities in the endothelial layer.
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